A vibration damping mechanism is broadly classified into a mechanism based on the mass law and a mechanism based on energy conversion. In the case of the method based on the mass law, a material large in mass, such as lead, is used, and a higher vibration damping effect can be obtained by using a material larger in mass. However, the method based on the mass law is limited in its application because products using this method are heavy.
On the other hand, the method based on energy conversion utilizes viscoelasticity of a polymer. Vibration damping with polymer utilizes the function of a polymer to convert external vibration energy to thermal energy and release generated heat into the outside to eliminate the vibration energy. However, such a vibration damping effect is obtained only at temperatures near the glass transition temperature (Tg) of the polymer. That is, a conventional vibration damping material using a polymer involves a problem that a usable temperature range within which the vibration damping material exhibits a high loss factor (tan δ) required for vibration damping is narrow.
Whereas, in order to broaden the temperature range within which vibration damping performance is exhibited, blending of two or more polymers having a large difference in glass transition temperature has been conventionally carried out. However, when polymers blended are incompatible, a loss factor peak (hereinafter, referred to as a “temperature peak”) is observed at the glass transition temperature of each of the polymers, and therefore a wide temperature peak cannot be achieved. On the other hand, when polymers blended are compatible, a single temperature peak is observed. Consequently, an attempt to design a semi-compatible polymer blend has been made. Such a semi-compatible polymer blend has a wider temperature peak, but involves a problem that the height of the temperature peak is lowered to deteriorate vibration damping performance.
Further, there are proposed a method in which an interpenetrating polymer network is formed by two or more polymers and a method in which a compatibilizing agent is added to incompatible resins (Japanese Patent Application Laid-open No. 2001-152028). According to these methods, a wider temperature peak can be achieved, but there is a problem that the height of the temperature peak is lowered to deteriorate vibration damping performance.
Furthermore, there are also proposed a method in which a low-molecular weight compound having three or more cyclic structures is added to a polymer (Japanese Patent Application Laid-open No. 5-65382) and a method in which a low-molecular weight compound that increases dipole moment is added (Japanese Patent Application Laid-open No. 9-302139). However, these methods involve a problem that crystallization or bleeding of the low-molecular weight compound occurs.
Further, there is also a method in which an inorganic filler is added to a polymer to convert vibration energy to frictional heat to dampen the vibration energy. However, this method involves a problem that the loss factor of the resin is lowered due to addition of the inorganic filler.
Further, there is also a method for reducing the weight of a material by forming a porous structure. A porous vibration damping material has improved sound absorbency, but the vibration damping performance thereof based on the mass law is lowered due to a reduced weight. In addition, pores in the porous vibration damping material do not contribute to energy conversion, and therefore the porous vibration damping material cannot have an increased loss factor.
Further, there is also proposed a vibration control method using antiphase vibration generated by an actuator (Japanese Patent Application Laid-open No. 7-26784, Japanese Patent Application Laid-open No. 6-158747, Japanese Patent Application Laid-open No. 8-61003). However, the method involves a problem that a large-scale device such as a speaker and a vibration generator are required.